Very high-T hydration in the lower gabbro section of Semail ophiolite, Hydrous components of the lower crust at fast-spreading ridge

Totally fresh black gabbros are observed locally at the very base of the gabbro section in southern Semail ophiolite. Devoid of medium to low-temperature diffuse alteration, the black gabbros are characterized by cloudy plagioclase marked by a high density of hydrothermal inclusions below the optical microscope resolution, evolving into diopside and pargasite crystals ∼10 μm sized. They record hydration at temperature over 900 °C as shown by their major elements composition. Focused Electron Backscattered Electron Diffraction (EBSD) study of the crystalline inclusions, suggests a phase transition from diopside to amphibole, and traces consistent crystallographic relationships of diopside inclusions with their host plagioclase. The detailed geometry of the microcrack system traces a complete fluid path from inter-grain cracking, to plagioclase intra-grain network, possibly thermally-induced. The evidence of solid-state deformation overprinting the magmatic textures suggests that twinning mechanism in plagioclase controls the intra-grain diffusion of hydrous fluids. Located at the interface of the Moho Transition Zone (MTZ) and semi-brittle lithosphere, the ‘black gabbros’ record a narrow domain where focused fluid flow induces rapid cooling below the gabbro solidus at the lowest limit of the magma chamber. •"Black gabbro" devoid of low-T alteration exposed locally, at the Moho limit.•Plagioclase is loaded with diopside/hornblende inclusions.•Major-elements and EBSD exploration of phase relationships.•Both datasets record transition from magmatic to ductile deformation.